Unconsolidated Carbonate Sediments Research Articles

Mapping and U[sbnd]Th dating of the world's deepest blue hole (South China Sea): Implications for its timing, possible volcanogenic origin, and Pleistocene eolianites in the Xisha Islands

The Sansha Yongle Blue Hole (SYBH) is the deepest (301.19 m) blue hole known in the world and is a water-filled karst opening in the intertidal reef flats of the eastern Yongle Atoll, Xisha Islands, South China Sea. It is differentiated from the surrounding environment by its deep blue colour as observed when viewed from above. The SYBH exhibits extremely rare karst networks and speleothems and differs from the blue holes of the Bahamas and Bermuda, indicating a different development process. Here, we present the morphology and UTh ages of the SYBH, which document the evolution of the coastal cave over time. Morphology and lithostratigraphy indicate that the hole resulted from dissolution via ascending acidic water/gas followed by subsequent collapse. The adjacent volcanoes surrounding the eastern Yongle Atoll are the most likely source of these ascending fluids. UTh ages of the cemented carbonate sand underlying the entrance constrain an upper limit of the opening time of the SYBH to be up to 111.7 ± 9.5 kyr BP. Deeply submerged speleothems at −116 m determine the lower collapse time of the SYBH to be 29.16 ± 0.17 kyr BP. The SYBH probably formed due to the upward release of volcanogenic acid water/gas through low-lying unconsolidated carbonate sediments, followed by buoyant collapses due to a 40-m fall in the sea level during 31–29 kyr BP. As the sea level rose to −15 m in the early Holocene, corals grew in the cave opening since 8.58 kyr BP and surrounded the entrance for the SYBH.

Holocene sediments of an inundated sinkhole: facies analysis of the “Great Blue Hole”, Lighthouse Reef, Belize

Anoxic sinkholes are unique geomorphological features of modern reef systems and in terms of “lake-like” sediment successions very special marine depositional environments. This study presents results of a sedimentological survey, which focuses on the “Great Blue Hole”, a circular Pleistocene karst cavity, situated in the shallow eastern lagoon of Lighthouse Reef Atoll (Belize, Central America). The investigated Blue Hole is located within the tropical cyclone belt and acts as consequence of its unique geomorphology as a giant sediment trap for both fair weather and storm sedimentation. As an important innovation step, we applied a multi-proxy approach based on textural, compositional and geochemical data to ensure a more reliable differentiation between sediments with fair weather and storm condition affiliation. Sediments having formed under both conditions were texturally categorized by determination of classical sedimentary parameters such as mean grain size and sorting. Sediment composition has been quantified in thin sections based on 40 samples discretely taken along a core from the bottom of the 125 m deep and 320 m wide cavern. Classification of sedimentary facies follows a modified scheme for unconsolidated carbonate sediments based on the Dunham nomenclature. Fair-weather sediments are generally finer than 20 μm and consist on average of 81% carbonate fine material, 12% Halimeda platelets, 3% coral fragments, 2% organic material, 1% mollusc shells, 1% foraminifer tests, and minor portions of coralline red algae and echinoderm plates (0–1%). Intercalated event bed sediments are coarse-silt to sand-sized (> 20 μm) and differ in terms of generally lower fine carbonate material amounts (50%) and increased abundances of over-washed Halimeda platelets (28%) and coral fragments (6%). Combined analyses of this study have revealed four primary sedimentary facies with affiliation either to the background (mudstone) or cyclonic event sedimentation (Halimeda wackestone, algae-rich packstone, coralgal rudstone) as well as a transitional sub-facies indicating storm-induced background overprinting (background wackestone). The striking compositional and textural differences of the Halimeda wackestone, algae-rich packstone and coralgal rudstone facies are considered to be another helpful (semi)-quantitative proxy to identify event beds of tropical cyclones in unconsolidated sinkhole sediment successions.

A Budget of Carbonate Framework and Sediment Production, Kailua Bay, Oahu, Hawaii

ABSTRACT Sediments of the bay and coastal plain of Kailua (Oahu, Hawaii) are > 90% biogenic carbonate produced by destruction of primary reef framework (coral and encrusting coralline algae) and by direct sedimentation through the biological activity of calcifying organisms (the green alga Halimeda, the branching coralline alga Porolithion gardineri, molluscs, and benthic foraminifera). Field measurements of benthic community structure, gross carbonate production, bioerosion, and direct sedimentation in 17 physiographic zones in Kailua Bay are used to calculate modern calcareous sediment production rates in the 12 km2 fringing reef system. Total gross carbonate productivity by corals and encrusting coralline algae (based on mapped percent cover and known growth rates) occurs at an average rate of 1.22 (± 0.36) kg m-2 y-1 over hard substrates of the reef platform, corresponding to 0.8 (± 0.2) mm y-1 (using a bulk density of 1.48 g cm-3, the average of coral and coralline algae). Coralline algae contributes 42% of the total gross productivity. Bioerosion of coral and coralline algae facies at and near the reef surface (estimated from slabbed reef samples) occurs at average rates of 0.10-1.15 kg m-2 y-1 and releases 1,911 (± 436) m3 of unconsolidated carbonate sediment annually. Mechanical erosion (coral breakage) likely contributes an additional 315 m3 y-1. Carbonate sediment is also produced directly by the green alga Halimeda, branching coralline algae, molluscs, and benthic foraminifera at a combined rate of 1,822 (± 200) m3 y-1 (using densities specific to sediment origin). The total rate of production of unconsolidated carbonate sediment in Kailua Bay is the sum of these sources, amounting to 4,048 (± 635) m3 y-1. Normalizing gross sediment production (in kg y-1) by reef habitat area (in m2) generates average rates of productivity (in kg m-2 y-1) that are directly comparable to each other and to the literature; such rates can also be employed and tested in other reef settings. In Kailua Bay, the total production of calcareous sediment corresponds to an average (normalized) rate of 0.53 (± 0.19) kg m-2 y-1, with 0.33 (± 0.13) kg m-2 y-1 contributed through erosion of the coralgal framework and 0.20 (± 0.06) kg m-2 y-1 contributed by direct sediment production on the reef surface. Applying these modern sediment-production rates over the 5,000 years that Kailua Bay has been completely inundated by postglacial sea-level rise, an estimated 20.2 (± 3.2) 106 m3 of unconsolidated carbonate sediment has been produced in the system. The volume of sediment stored in the various reef channels and holes in Kailua Bay is 3.7 (± 0.3) 106 m3, or 19% of that produced since 5,000 y BP. The volume of sand in the modern beach is 1.0 (± 0.1) 106 m3, or 5% of Holocene sediment production. The volume of carbonate sediment stored in the coastal plain is estimated, using core-log data and associated radiocarbon ages, to be 10.0 (± 1.8) 106 m3, or 51% of Holocene sediment production. The remaining 25% likely represents sediment loss due to the natural processes of dissolution, attrition, and transport offshore. These export terms are not well understood and emphasize the need for sediment dynamics to be incorporated into reef and sediment budgets. Although sediment production in this reef system is prodigious, the rate of new sediment supplied to the beachface is less than 2% of what moves on and off the beach seasonally.

Application of Electron Microprobe Scanning Techniques to Resolve Trace Element Variations in Biogenic Aragonite.

Aragonite (CaCO3) is one of the principal carbonate minerals utilized by metazoans to form shells, tests and other skeletal elements. It may also be inorganically precipitated under certain geological conditions. Aragonite is unstable in meteoric diagenetic environments (relative to its polymorph calcite), and is prone to dissolution and/or replacement by other minerals (usually calcite). The mobilization of Ca2+ and CO32− during aragonite alteration is largely responsible for early diagenetic cementation and the formation of limestone from unconsolidated carbonate sediment. Our study has been investigating the very earliest stages of aragonite alteration. To date, we have limited our work to bivalves of the genera Donax, Chione and Austrovenus collected from, respectively, southern Alabama, southwestern Florida and eastern North Island, New Zealand. Chione and Austrovenus are closely related genera which should permit direct comparison of results. The Florida and New Zealand study locations were chosen because they were near fossil localities containing identical Tertiary-aged specimens that had been subjected to more prolonged alteration. Our results confirm that initial aragonite alteration in meteoric environments is driven by dissolution. Outer surfaces begin to dissolve very rapidly (laboratory experiments suggest that this may occur after only a few months of exposure to meteoric water); leading to a chalky consistency along the outer extremities of the shell. Simultaneous selective leeching of some trace elements (notably Sr2+) also occurs. More prolonged dissolution results in progressive loss of mass and Sr2+ as well as penetration of the chalky “zone” into the interior of shells. Highly altered shells consist almost entirely of chalk.

Geoacoustic and physical properties of carbonate sediments of the Lower Florida Keys

Near-surface sediment geoacoustic and physical properties were measured from a variety of unconsolidated carbonate sediments in the Lower Florida Keys. Surficial values of compressional and shear speed correlate with sediment physical properties and near-surface acoustic reflectivity. Highest speeds (shear 125–150 m s-1; compressional 1670–1725 m s-1) are from sandy sediments near Rebecca Shoal and lowest speeds (shear 40–65 m s-1; compressional 1520–1570 m s-1) are found in soft, silty sediments which collect in sediment ponds in the Southeast Channel of the Dry Tortugas. High compressional wave attenuation is attributed to scattering of acoustic waves from heterogeneity caused by accumulation of abundant shell material and other impedance discontinuities rather than high intrinsic attenuation. Compared to siliciclastic sediments, carbonate sediment shear wave speed is high for comparable values of sediment physical properties. Sediment fabric, rather than changes due to the effects of biogeochemical processes, is responsible for these differences.

Rapidity of freshwater calcite cementation—implications for carbonate diagenesis and sequence stratigraphy

In less than 10 years, subaerially-exposed Holocene oolitic sand on Eleuthera Island, Bahamas was completely case-hardened by freshwater vadose calcite cementation. This phenomenon demonstrates the rapidity of freshwater carbonate diagenesis and explains, in part, why carbonate platforms preferentially shed unconsolidated sediment during highstands and not during lowstands when they are subaerially exposed. It further proves, contrary to what the Exxon conceptual sequence stratigraphic models imply, that carbonate deposits do, in fact, respond differently to subaerial exposure than siliciclastic deposits. Case hardening of unconsolidated carbonate sediment in response to subaerial exposure and early, near-surface freshwater diagenesis helps explain why shallow-water platform sediments are preferentially shed into adjacent basins during highstands of sea level and not during lowstands. During lowstands, rapid subaerial lithification of carbonate sediments retards their reworking by wind or water. Once lithified, these deposits are further stabilized by vegetation and karstification, inhibiting their transport as unconsolidated sediment into adjacent deeper-water basins. Observations from Eleuthera Island also document the reciprocal processes associated with freshwater carbonate diagenesis. Calcium carbonate released by partial dissolution of aragonitic ooids was locally reprecipitated as pore-filling, low-magnesian calcite cement. This reciprocity is confirmed by the confinement of freshwater dissolution and cementation to the upper 10–20 cm of the subaerially-exposed body of oolitic sand. Below this case-hardened surface, oolitic sands remained unconsolidated. Only very minor amounts of calcite cement were required for effective casae hardening.

A test of a simple model of the acoustic velocity in marine sediments

A simple physical model is proposed for the dependence of the acoustic velocity of marine sediments on the porosity and lithology. The model is tested for clay-rich marine sediments using data from the Deep Sea Drilling Project, but has also been successfully used to calculate the acoustic velocity in consolidated and unconsolidated carbonate sediments. The model is a combination of the Wood [A. B. Wood, A Textbook of Sound (Macmillan, New York, 1941)] and Wyllie [Wyllie et al., Geophysics 21, 41–70 (1956)] (time average) models: Any medium may be thought of as composed of one fraction that is comparable to a slurry of particles in suspension and a second that consists of a rigid grain matrix. As the porosity decreases, so too does the fraction of slurry that makes up the sample. A slurry has essentially no shear strength, and thus the bulk compressibility is expressed as the weighted mean compressibility of the solid and fluid components. The rigid matrix velocity is determined using the time average equation. The variation of the grain (zero-porosity) velocity with lithology is similarly represented using the time average equation. The net bulk velocity is then taken to be the inverse of the weighted mean travel time for each of the component fractions. This approach could simplify the modeling of acoustic velocity variations given an independent measure of the porosity, or vice versa.

Synsedimentary Chert Breccia: A Mississippian Tempestite

ABSTRACT A section of crinoidal calcarenites of the Lower Burlington Limestone, Middle Mississippian, displays an occurrence of chert nodules penecontemporaneously brecciated by a short-lived and localized high-energy event interpreted as the touch-down of a tornado system. This example of tempestite demonstrates the syngenetic generation of completely indurated and fragile chert nodules within an unconsolidated carbonate sediment. It is speculated that such an early origin of cherts is more widespread than generally assumed.

Calcite Cements in the Corallian Beds (Upper Oxfordian) of Southern England

ABSTRACT Three generations of sparry calcite, defined by variations in fabric and their reaction with artificial stains, occupy primary and secondary voids in the Corallian Beds of southern England. They appeared in the order; non-ferroan calcite, fibrous ferroan calcite, granular ferroan calcite. Non-ferroan calcite has a restricted distribution and is suggested to he the result of exposure of the unconsolidated carbonate sediments to the subaerial environment. The ferroan calcites are widespread; they were precipitated after the sediments had become sufficiently buried to isolate them from the subaerial and submarine environments. Some of the calcite was produced from calcium carbonate released by dissolution of skeletal aragonite and some from grain contact solution, but other sources must lso have been involved. Similar diagenetic sequences are present elsewhere in the Jurassic of southern England.

Sedimentary environments and unconsolidated carbonate sediments of the fringing coral reefs of Mahé, Seychelles

Extensive fringing coral reefs occur around Mahé: the largest island in the Seychelles Archipelago. Several sedimentary environments, which are best developed on reefs exposed to the Southeast Monsoon (the dominant wind), are associated with the reefs. These are: the beach; rippled sand zone; marine grass bed: radial zone; algal ridge; reef edge; reef slope; “plateaux”; mangrove and former mangrove swamps; deltas; channels; the fore-reef environment. Skeletal carbonate sands form a veneer on the reef flats and are widespread in the fore-reef environment and locally they pass into quartz sands. Skeletal debris from sessile organisms in the reef flat deposits is only abundant landward of the areas in which they live: indicating that current transportation is dominantly landward, with little material being carried from the reef flats into the fore-reef and channel environments. It is often difficult to relate the grain-size measures of the sediments to their environment of deposition. The turbulent conditions in the beach, algal ridge and reef edge environments are reflected in the absence of fine material and the low dispersion values of the associated sediments. Grass bed samples have high dispersion values and are relatively rich in fines trapped by the grasses. Radial zone samples show much variation in grain-size measures due to the diversity of the conditions of deposition. The skewness in mangrove swamp sediments is positive, indicating normal current deposition. Sheltered fore-reef deposits tend to be rich in fines and to have positive skewness values due to the quiet conditions, whereas fine material is winnowed from windward fore-reef sediments.

Orthoconic nautiloids as indicators of shoreline surface currents

ABSTRACT The vertical orientations of shells of Rhynchorthocerasin Lower Ordovician limestone at Satatorp, Vastergotland, Sweden indicate that these shells stranded in unconsolidated carbonate sediment in shallow water. The mean position for the siphuncle, which takes up 1/6 of the diameter of the shell, is at 267° (almost directly in the bottom location) with an angular deviation of 27°. It is suggested, that the shells have remained undisturbed by swash action sufficiently long to become securely fastened in the sediment. This suggests the possibility of using the orientation of the siphuncle of suitable orthoconic nautiloids as an indicator of rolling action by swash currents after stranding.

Porosity Changes During Lithification from Unconsolidated Carbonate Sediment to Consolidated Limestone: ABSTRACT

As carbonate sediments are lithified to limestone, two major porosity changes commonly occur. First, interstitial pore space is partly or, less commonly, totally obliterated as the grains are bound together; second, moldic porosity usually is developed. In sediments devoid of CaCO3 mud, porosity development tends to be more pronounced than porosity elimination; yet preliminary studies indicate that, where CaCO3-mud matrix is found, the moldic pore space is more likely to be retained. Moldic porosity is formed by the dissolution of aragonitic grains which are abundant in shallow-water marine sediments, but is not formed at the expense of grains composed of the two types of calcite (high-magnesian and low-magnesian). Aragonite and both types of calcite are synthesized predominantly by organisms. Because different biologic groups selectively synthesize carbonate material of either aragonite or one of the two types of calcite, a predictable relation exists between biological activity and the tendency to form moldic porosity. The tendency for aragonitic grains to develop porosity is variable. Shell material is more strongly affected by dissolution and consequent moldic porosity development than are the more finely crystalline ooids, which may resist dissolution. Yet, ooids are more likely to form moldic (oomoldic) porosity than are pelletal and cryptocrystalline grains. Where the process of diagenesis continues, the newly created pore spaces are occluded by drusy calcite mosaic. End_of_Article - Last_Page 614------------

Radiocarbon Dating of Size Fractions of a Sample of Bahama Carbonate Sediment

ABSTRACT Radiocarbon ages have been determined for five grain-size fractions of an unconsolidated carbonate sediment from Andros Island, Bahamas. These ages along with carbonate mineralogy of three size fractions suggest that some old, perhaps Pleistocene, carbonate composes part of the sediment.

Direct Printing of Contour Maps of Facies Data by Computer: ABSTRACT

A Burroughs 220 computer is being used at Stanford for fitting contour maps to facies data. The computer fits a smoothly curving parabolic surface to the data and then prints a contour map of the surface. Human error in fitting the contours is virtually eliminated. The method is particularly useful where direct contouring of data values is difficult or meaningless because of appreciable random fluctuations and error. The map printed by the computer smooths over the irregularities and shows the general trends. The contoured surface is fitted so that the sum of the squares of the numerical differences of the actual data above and below the surface is the least possible. This method has been useful in mapping facies variations in a Pennsylvanian limestone bed in New Mexico and variations of modern unconsolidated carbonate sediments in the Gulf of Mexico and adjacent coastal swamps of the Everglades in southern Florida. Ratios, percentages, and average grain diameters mapped in these applications revealed systematic trends that were previously obscure. Areas mapped by this method must be rectangles, but sampling localities, which may be measured stratigraphic sections, oil wells, or places where sediment samples were obtained, can be arranged irregularly within a rectangle. An arbitrary geographic coordinate system is established so that each locality is described by two coordinate values. The coordinate units may be feet, miles, or, as has been proved convenient, tenths of an inch scaled off the map. The coordinate values and the value to be contoured for each locality form the basic data fed to the computer. The general instructions for the method are fed from paper tape into the computer's memory before the computation begins. In addition, the computer must be instructed as to the contour interval and the dimensions of the map that it is to print. The contours printed out by the computer consist of individual bands of a letter or symbol, such as A, B, and $. Each band spans one contour interval, and blank bands alternate with printed bands. End_of_Article - Last_Page 268------------